It is known to catalytically reduce NOx in exhaust gas of an internal combustion engine to N2 with a suitable reductant. Three examples are selective catalytic reduction (SCR), lean-NOx catalysis and NOx-trap regeneration.
In SCR, the reductant is typically a NOx-specific reactant. By “NOx-specific reactant” herein, we mean a reducing agent that, in most conditions, preferentially reduces NOx over other components of a gaseous mixture. Examples of NOx-specific reactants include nitrogenous compounds such as nitrogen hydrides, e.g. ammonia (NH3) or hydrazine as such or by way of a NH3 precursor.
By “NH3 precursor” we mean one or more compounds from which NH3 can be derived, e.g. by hydrolysis. These include urea (CO(NH2)2) as an aqueous solution or as a solid or ammonium carbamate (NH2COONH4). If the urea is used as an aqueous solution, a eutectic mixture, e.g. 32.5% urea (aq), is preferred. Additives can be included in the aqueous solutions to reduce the crystallisation temperature.
Known SCR catalysts include Pt-based catalysts, which can catalyse the reduction of NOx with NH3 at between about 175° C. and about 250° C., medium temperature vanadium-based catalysts e.g. V2O5/TiO2, which operate in the temperature range between about 260° C. and about 450° C. and zeolite-based catalysts which function with increasing activity at increasing temperature.
Several chemical reactions occur in the NH3 SCR system, all of which represent desirable reactions which reduce NOx to elemental nitrogen. The overall desired reaction is represented in equation (1).4NO+4NH3+O2→4N2+6H2O  (1)
Competing, non-selective reactions with oxygen can produce secondary emissions or may unproductively consume NH3. One such non-selective reaction is the complete oxidation of NH3, represented in equation (2).4NH3+5O2→4NO+6H2O  (2)
It will be appreciated that at lower temperatures, below about 100-200° C., NH3 can also react with NO2 to produce an explosive mixture of ammonium nitrate (NH4NO3) and ammonium nitrite (NH4NO2). For the avoidance of doubt, the present invention does not embrace such reactions or the promotion of conditions which bring them about. For example, the reaction can be avoided by ensuring that the temperature does not fall below about 200° C. or by supplying into a gas stream less than the precise amount of NH3 necessary for the stoichiometric reaction with NOx (1 to 1 mole ratio).
Urea hydrolyses at temperatures above 160° C. according to equation (3) to liberate NH3 itself. It is also believed to decompose thermally at this temperature and above according to equations (4) and (5) resulting in reduction of NOx, as evidenced by formation of CO during SCR processes with urea (see SAE 900496 and SAE 930363 (both incorporated herein by reference)).CO(NH2)2+H2O→2NH3+CO2  (3)CO(NH2)2→.NH2+CO  (4).NH2+NO→N2+H2O  (5)
Lean-NOx catalysts (LNCs) are sometimes also referred to in the literature as lean-NOx reduction catalysts, “DeNOx catalysts” and NOx occluding catalysts.
In lean-NOx catalysis, hydrocarbons (HC) react with nitrogen oxides (NOx), rather than oxygen (O2), to form nitrogen (N2), carbon dioxide (CO2) and water (H2O) according to reaction (6).{HC}+NOx→N2+CO2+H2O  (6)
The competitive, non-selective reaction with oxygen is given by reaction (7).{HC}+O2→CO2+H2O  (7)
There are two preferred groups of LNC to selectively promote the desired reaction (6) described in the literature: platinum (Pt) on alumina (Al2O3) and copper (Cu)-substituted zeolite such as Cu/ZSM-5.
A typical NOx-trap formulation includes a catalytic oxidation component, such as Pt, a NOx-storage component, such as compounds of alkali metals e.g. potassium and/or caesium; compounds of alkaline earth metals, such as barium or strontium; or compounds of rare-earth metals, typically lanthanum and/or yttrium; and a reduction catalyst, e.g. rhodium. One mechanism commonly given for NOx-storage during lean engine operation for this formulation is that, in a first step, the nitric oxide reacts with oxygen on active oxidation sites on the Pt to form NO2. The second step involves adsorption of the NO2 by the storage material in the form of an inorganic nitrate.
When the engine runs intermittently under enriched conditions or at elevated temperatures, the nitrate species become thermodynamically unstable and decompose, producing NO or NO2. Under rich conditions, these nitrogen oxides are reduced by carbon monoxide, hydrogen and hydrocarbons to N2, which can take place over the reduction catalyst.
Whilst the inorganic NOx storage component is typically present as an oxide, it is understood that in the presence of air or exhaust gas containing CO2 and H2O it may also be in the form of the carbonate or possibly the hydroxide. We also explain in our WO 00/21647 (incorporated herein by reference) that NOx-specific reactants can be used to regenerate a NOx-trap.
EP-B-0341832 (incorporated herein by reference) describes a process for combusting particulate matter in diesel exhaust gas, which method comprising oxidising nitrogen monoxide in the exhaust gas to nitrogen dioxide on a catalyst, filtering the particulate matter from the exhaust gas and combusting the filtered particulate matter in the nitrogen dioxide at up to 400° C. Such a system is available from Johnson Matthey and is marketed as the CRT®.
For the purposes of the present specification, generally we refer to methods of catalytic reduction of NOx to N2 in exhaust gases of internal combustion engines with a suitable reductant as NOx-reduction methods and to catalysts for promoting the reduction of NOx to N2 as NOx-reduction catalysts. Such catalysts include SCR catalysts, lean-NOx catalysts and NOx-traps.
A problem with the above NOx reduction methods is to control the addition of the reductant. If too little reductant is added, NOx reduction may be inadequate to meet an emission standard. If too much reductant is added this can cause a number of problems. For example, if the reductant is ammonia, its release into the atmosphere is undesirable because it is a biological poison and it has an unpleasant odour. Whilst excess ammonia can be oxidised using a suitable catalyst downstream of the NOx-reduction catalyst, this produces NOx, thus defeating the very purpose of the NOx reduction method. Hydrocarbon fuels, e.g. diesel or gasoline, are also legislated components of exhaust gas and so emission of excess hydrocarbon reductant can cause the system to fail a relevant emission standard.
Systems to control reductant addition are known, but tend to require very complicated control regimes involving multiple sensor inputs and processors to run complex algorithms. As a result, such systems are very expensive.
US-A-2002/0194841 (incorporated herein by reference) discloses a method of reducing NOx emissions from vehicular diesel engines by an external reductant supplied to a SCR system including a reducing catalyst, which method comprising the steps of sensing one or more engine operating parameters, such as speed and torque from a speed/load sensor, to predict a concentration of NOx emissions indicative of the actual quantity of NOx emissions produced by the engine when the catalyst temperature is within a set range and metering the external reductant to the catalyst at a rate sufficient to cause the catalyst to reduce the calculated concentration of NOx emissions.
JP-A-2002-122019 (incorporated herein by reference) discloses a method of preventing thermal degradation in a NOx-trap by detecting the temperature in the NOx-trap and regulating reductant addition to maintain the NOx-trap temperature within a pre-determined range.
DE-A-9913268 (incorporated herein by reference) discloses a system for monitoring the efficiency of a NOx reduction catalyst in a lean burn engine comprising a fuel feed device for dispensing a predetermined quantity of fuel into exhaust gas upstream of the catalyst to make available an amount of chemical energy and, dependent on catalyst efficiency, to provide an amount of thermal energy, flow and temperature sensors for measuring the thermal energy into and out of the catalyst and a data processing unit communicating with the fuel feed device and the temperature sensors, which constructs an energy balance for the catalyst, and hence provides a correlation signal indicating the performance of the catalyst.
In JP-A-62-117620 (incorporated herein by reference), there is described a method removing nitrogen oxides in gasoline engine exhaust gas employing two NOx-traps arranged in parallel wherein the NOx-traps are used alternately to absorb NOx from the exhaust gas under the control of a two-way valve. The off-line NOx-trap is regenerated using a suitable reductant such as hydrogen, ammonia, carbon monoxide or methane.
“Development and evaluation of a DeNOx system based on urea SCR”, by Martin Elsener et al., MTZ worldwide, November 2003, Volume 64, p. 28-31 (incorporated herein by reference) describes the use of a NOx sensor which is cross-sensitive against ammonia to provide feedback control of reductant delivery in an exhaust system including a SCR catalyst.
We have investigated methods of calibrating reductant addition and of controlling reductant addition by feedback. We have now devised a number of simple methods and systems that are cheap and effective at reducing reductant-based emissions. Systems embodying these methods are particularly relevant to the retrofit market.